UI for an MMO is an incredibly important thing. The fact that one of the top five complaints about this game was it's horrid UI, and the fact that FFXI purists always complain about this game's UI becoming more "WOW"-like, should be enough of an indication of its importance.

UI design is one of those things that developers can easily and safely crowdsource by letting people develop addons, and then reacting to and integrating things from the UI development community. This is something that the WOW devs realized and have done from day one, and it's why WOW's UI is still the standard/baseline UI for pretty much every MMO. If they hadn't let people develop addons, its UI would have been horribly outdated by now and would be one of the game's biggest weaknesses.